Method for evaluating delayed fracture of metallic materials

Abstract

In the method for evaluating the delayed fracture characteristics of a metallic material, on the end face, a solution holding material containing a solution with a pH of 3.5 or more and containing a chloride is disposed on the end face of the metallic material, and the state of maintaining the deliquescence humidity of the chloride is continued to corrode the end face.

Description

Technical Field

[0001] The present invention relates to a delayed fracture evaluation method for evaluating the delayed fracture characteristics of a metal material used in a non-dry corrosive environment in the end face of the metal material. Background Art

[0002] With the increase in the strength of automotive part materials, there occurs a so-called "delayed fracture" phenomenon in which mechanical properties such as elongation deteriorate when hydrogen penetrates into the material. It is known that the occurrence of delayed fracture is caused by an increase in the amount of hydrogen penetrating into the material, and the delayed fracture sensitivity increases as the strength and load stress of the material increase. In particular, in the manufacture of automotive parts, usually, steel sheets as raw materials (blank sheets) are often trimmed into a specific shape by shearing or subjected to punching to be used, so delayed fracture at the end face of the processed metal material becomes a problem.

[0003] Conventionally, evaluation methods for delayed fracture have been carried out by introducing hydrogen through acid immersion tests, cathodic charging tests, corrosion tests, etc. Non-Patent Document 1 describes a technique for evaluating delayed fracture characteristics by introducing hydrogen into a material by immersing it in an aqueous hydrochloric acid solution. Patent Document 1 describes a technique for simply evaluating delayed fracture characteristics by introducing hydrogen into a stressed thin steel sheet through a cathodic charging test.

[0004] In Patent Document 2, as a method for simply evaluating the delayed fracture characteristics of a metal material used in an atmospheric corrosion environment, a technique for evaluating the delayed fracture characteristics of a material accompanied by corrosion by simulating the day-night wet-dry process occurring in an atmospheric corrosion environment is described. In addition, in Patent Document 3, as an electrochemical corrosion resistance evaluation method using a water-containing material focusing on the shape of a test object as an automotive part, a laboratory corrosion resistance evaluation method for reducing the influence of surface unevenness by a muddy water-containing material is described.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2005-134152

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2016-180658

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 2019-32173

[0010] Non-Patent Documents

[0011] Non-Patent Document 1: Iron and Steel, Vol. 79, No. 2, pages 227-232 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] As described above, delayed fracture often occurs in parts subjected to shearing or hole-opening processes, and it is necessary to evaluate the delayed fracture of the end face of the metal material after such processing. The evaluation methods described in Non-Patent Document 1, Patent Document 1, and Patent Document 3 can also perform tests on the end face at the same time. However, the tests in Non-Patent Document 1 and Patent Document 1 are not tests considering the corrosion environment in a wet state such as snow, precipitation, or splashing water, so it is difficult to judge the delayed fracture characteristics in the actual environment.

[0014] The premise of Patent Document 2 is to conduct tests in an environment where day-night wet-dry behavior occurs, and directly evaluate the delayed fracture characteristics of the material. However, an automobile is used in various environments, and Patent Document 2 does not envision use in a snowfall area or an environment where wading occurs. The purpose of Patent Document 3 is to use a water-containing material to penetrate the surface treatment film, and its concept is different from the maintenance of the liquid film thickness of the present invention.

[0015] The present invention is proposed in view of the above circumstances, and aims to provide a method for evaluating the delayed fracture characteristics of the end face of a metal material in a corrosion environment with a wet state of snow, precipitation, or splashing water with high precision.

[0016] Means for Solving the Problem

[0017] The present invention is proposed to achieve the above object, and its gist is as follows.

[0018] [1] A method for evaluating the delayed fracture characteristics of a metal material, which is a method for evaluating the delayed fracture characteristics of the end face of a metal material,

[0019] wherein, on the end face, a solution holding material containing a solution with a pH of 3.5 or more and containing a chloride is disposed on the end face, and the state maintained at the deliquescence humidity of the chloride is continued to corrode the end face.

[0020] [2] The method for evaluating the delayed fracture characteristics of a metal material according to [1], wherein the corrosion is continued while maintaining the liquid film thickness of the solution at 10 μm or more and 2500 μm or less.

[0021] [3] The method for evaluating the delayed fracture characteristics of a metal material according to [1] or [2], wherein the corrosion is carried out at a test temperature of -50 to 60°C.

[0022] [4] The method for evaluating the delayed fracture characteristics of a metal material according to any one of [1] to [3], wherein after supplying a solution with a pH of 3.5 or more and containing a chloride to the end face, the solution holding material is disposed on the end face.

[0023] [5] The evaluation method for the delayed fracture characteristics of a metallic material according to [4], wherein the supply of the solution is carried out by any one of spraying, sprinkling, dropping of liquid droplets, or dipping for less than 15 minutes.

[0024] [6] The evaluation method for the delayed fracture characteristics of a metallic material according to any one of [1] to [5], wherein the metallic material is a steel sheet having a tensile strength of 1180 MPa or more.

[0025] Effects of the Invention

[0026] According to the present invention, it is possible to accurately evaluate the delayed fracture characteristics of the end face of a metallic material in a corrosive environment with a wetting state of snow, precipitation, or splashing water. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a schematic view showing an example of a test piece used in the delayed fracture evaluation method.

[0028] Figure 2 is a schematic view showing an example of a test piece used in the delayed fracture evaluation method.

[0029] Figure 3 is a schematic view showing an example of a test piece used in the delayed fracture evaluation method. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, embodiments of the present invention will be described. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit its application or its use. The inventors of the present application evaluated the delayed fracture characteristics of high-strength steel sheets for automobiles in various actual environments. As a result, it was clarified that delayed fracture is likely to occur particularly in an environment where snow-melting salts are scattered. The reason is known to be that the wetting state of the steel sheet surface is maintained due to the attachment of snow and mud rolled up by the running of the automobile, and as a result, the corrosion condition is the most severe.

[0031] It is known that when the surface and the end face of a metallic material are placed in the same corrosive environment, the delayed fracture environment of the end face is more severe than that of the surface of the metallic material. In addition, compared with a test piece in which the end face of the metallic material is sealed, more cracking occurs in the test piece with the end face exposed. Therefore, it was confirmed that in addition to the evaluation of the delayed fracture characteristics of the surface of the metallic material, it is also necessary to conduct a test for evaluating the delayed fracture characteristics of the end face in a corrosive environment with a wetting state of snow, precipitation, or splashing water.

[0032] In addition, it is known that not only the surface of the metallic material but also the end face is kept in a wet state due to the attachment of snow and mud. Based on this result, it can be seen that a uniform wetting state must also be maintained on the end face in the evaluation test.

[0033] As described above, the method for evaluating the delayed fracture of the metal material of the present invention is carried out as follows: On the end face of the metal material, a solution holding material containing a solution with a pH of 3.5 or more and containing chloride is disposed on the end face, and the state maintained at the deliquescence humidity of chloride is continued to corrode the end face. In the following embodiments, the case of evaluating through the following steps is illustrated: (1) A supply step of supplying a solution with a pH of 3.5 or more and having chloride to the end face of the metal material; and (2) A corrosion step of maintaining the end face to which the solution is supplied in a state of maintaining a liquid film of the solution in an environment with a chloride deliquescence humidity or more at a test temperature of -50 to 60°C, thereby corroding the metal material.

[0034] First, in order to specifically evaluate the delayed fracture characteristics, it is necessary to make the metal material have an end portion formed by processing. As the processing method, for example, shearing processing, punching processing, laser processing, etc. can be cited. In addition, in the evaluation of the delayed fracture characteristics, not only a method of evaluating using the residual stress existing after processing can be cited, but also a method of fixing in a shape with stress applied using bolts, etc. can be cited.

[0035] In particular, the metal material to be evaluated is a steel material such as a steel plate with a tensile strength TS of 1180 MPa or more, but it is not limited thereto, and other metal materials such as Ti and Al can also be used. In addition, metal materials with plating are also included.

[0036] (1) Supply step

[0037] The supply step is a step of supplying a solution with a pH of 3.5 or more and having chloride to the end face of the metal material. If the pH of the solution is lower than 3.5, the dissolution of the metal material is promoted, and the hydrogen ions in the solution accompanying the dissolution of the metal material are reduced, thereby promoting the intrusion of hydrogen into the metal material. That is, if the pH of the solution is lower than 3.5, it becomes a test in an environment more severe than the actual environment, and the test of the delayed fracture characteristics in the actual environment cannot be carried out with high precision. Therefore, a solution with a pH of 3.5 or more is used. It should be noted that since the solution is neutral in the corrosion environment of the actual environment, it is preferable that the pH of the solution is 5 to 9.

[0038] As the corrosion environment of the actual environment, chloride is a common factor, so the solution contains chloride. It should be noted that when the end face of the metal material is only covered with snow, the snow does not contain chloride, but considering that the end face of the metal material will be affected by snow melting salts even when covered with snow. Therefore, the solution contains chloride. At this time, chloride is contained in the solution in the form of chloride ions.

[0039] The chloride ions represent Cl ions in salts containing Cl ions such as NaCl, MgCl2, CaCl2, etc. The solution contains one or more components of the above-mentioned chlorides. The solvent can either contain only chlorides such as NaCl, MgCl2, CaCl2, etc. in water, or can contain multiple chlorides, or can contain components other than chlorides. Examples of components other than chlorides include sulfides contained in the environment, nitrate compounds, urea mixed and scattered in snow melting agents, etc., but are not limited thereto. Considering the actual environment, it is preferable that the solution attached to the metal material is mainly composed of NaCl, such as brine.

[0040] A solution is a liquid-state mixture composed of two or more substances. An aqueous solution can be used, for example, with a liquid component containing water as the solvent and the above-mentioned chloride as the solute. In particular, for the solution used in the delayed fracture test, the chloride accounts for 50 to 100% by weight in the solute, preferably 70% or more. In this way, by increasing the weight ratio of the chloride, the management of the deliquescence humidity described later can be made simple. In addition, as for the weight percentages of the solute and the solvent, the concentration is arbitrary as long as there is no operational obstacle. For example, when NaCl accounts for more than half of the volume of the solution, NaCl precipitates due to being above the saturation amount. In such a case, it is not suitable for the present invention because the solution cannot be supplied uniformly.

[0041] The method of supplying the solution to the end face of the metal material is not particularly limited. As a specific example, there can be mentioned the dipping method (less than 15 minutes, preferably less than 10 minutes) in which the test piece is immersed in the solution and then taken out to attach the solution to the surface of the test piece, the method of coating the solution on the metal material by spraying or showering, the method of attaching the solution by spraying, the method of dropping the solution in a specified amount of droplets using a pipette, the method of arranging a solution holding substance such as a gauze containing the solution on the end face, etc. If the solution attachment based on dipping exceeds 15 minutes, corrosion occurs in the solution, which is not suitable because it is different from the corrosion mode of the present invention. It should be noted that spraying means spraying a solution with droplet diameters of 50 μm or more, and spraying means spraying a solution with droplet diameters less than 50 μm.

[0042] In addition, the solution supply in the supply process can also be carried out by arranging a solution holding substance such as a gauze containing the solution used in the corrosion process on the end face. In this case, the corrosion process described later is carried out using the solution holding substance arranged in the supply process. Thereby, the actual corrosion environment with unchanged state from the supply process to the corrosion process can be reproduced, and the accuracy of the test can be improved.

[0043] (2) Corrosion process

[0044] The corrosion process is a process of disposing a solution-containing solution holding substance on the end face of a metal material and maintaining a state in which the deliquescence humidity of the chloride is maintained, thereby corroding the end face of the metal material.

[0045] In the corrosion process, it is necessary to continuously maintain the solution on the end face of the metal material. Since the liquid film thickness has a great influence on the delayed fracture characteristics, it is an important factor. As a result of actually changing the liquid film thickness and evaluating the delayed fracture characteristics at each liquid film thickness, it can be seen that as long as there is at least a liquid film on the end face (a state greater than 0 μm), the delayed fracture characteristics can be evaluated. It should be noted that the liquid film thickness is obtained based on the result of converting the value obtained from the ACM sensor (manufactured by Syrinx Co., Ltd.) into a liquid film. In particular, it is preferable that the liquid film thickness is continuously maintained in a state of 10 μm to 2500 μm. When the liquid film thickness is less than 10 μm, the liquid film is not sufficiently formed and no corrosion occurs compared with the actual environmental test. Therefore, there are cases where the ultimate load stress is the same as the actual environment, but the corrosion mode (corrosion type) is different from the actual environment. Similarly, even when the liquid film thickness exceeds 2500 μm, there are cases where the ultimate load stress is the same as the actual environment, but the corrosion mode (corrosion type) is different from the actual environment due to the excessive thickness of the liquid film. Therefore, the liquid film thickness is 10 μm to 2500 μm. It should be noted that the liquid film thickness in the corrosion process is mainly based on the supply amount of the solution in the above supply process and the chloride amount management described later.

[0046] The test temperature in the corrosion process is not particularly limited, and it is carried out in an environment with a test temperature of, for example, -50 to 60°C. This test temperature is determined based on the measurement in the actual environment. In an environment where snow melting salt is scattered, it has been confirmed that there is an environment reaching -50°C. In addition, when a car is parked in direct sunlight, the body parts become as high as 60°C. Therefore, the temperature range of the present invention is set to -50 to 60°C. As a range that can be implemented by a general device, it is preferable that the test temperature is -20 to 40°C.

[0047] In the corrosion process, the relative humidity of the test environment is one of the important factors. According to multiple experiments, in the corrosion process, it is necessary to continuously maintain the state where the solution on the end face does not dry. The non-drying wet state means that a liquid film is continuously formed on the end face of the metal material (preferably the liquid film thickness is 10 μm or more). If the liquid film is interrupted, the chloride amount on the test object part becomes uneven, which is not preferable because of the large difference from the actual environment. In addition, in order to continuously form a liquid film without interruption, the wetting condition must be continuous and the shaking is small.

[0048] Therefore, the corrosion process is carried out in an environment with a humidity above the deliquescence humidity of the chloride. The deliquescence humidity is the humidity at which wetting occurs on the surface of the test piece due to the hygroscopicity (deliquescence) of the chloride. Thus, the solution on the end face can be maintained in a non-drying state. The deliquescence humidity is determined by the chloride supplied to the end face, i.e., the type of chloride in the solution. For example, when using a salt mainly composed of NaCl, the relative humidity is 75% RH or above; when using a salt mainly composed of MgCl2, the relative humidity is 33% RH or above; when using a salt mainly composed of KCl, the relative humidity is 84% RH or above, and it is uniformly managed without humidity change. It should be noted that the change in the liquid film thickness during the corrosion process can be tolerated within ±10% of the set value. If it exceeds this value, uneven results will occur and it is not suitable.

[0049] As described above, the amount of chloride in the solution and the relative humidity of the test environment are amounts that can maintain the state where the liquid film thickness exists without interruption. That is, the liquid film thickness is determined by the relative humidity of the test environment and the amount of chloride. If the amount of chloride is too much or too little, it is difficult to reproduce the actual environment. The preferred amount of chloride to ensure the above liquid film thickness is 1000 - 200000 mg / m 2 。

[0050] For example, in an environment with a temperature of 25°C and a humidity of 95% RH, when the amount of chloride is 0.1 g / m 2 , the liquid film thickness is about 10 μm; when the amount of chloride is 1 g / m 2 , the liquid film thickness is about 100 μm. It should be noted that from the perspective of managing the supplied amount of chloride and relative humidity, considering the liquid film thickness, the preferred liquid film thickness for the corrosion process is 40 - 1500 μm. Additionally, it is more preferable that the relative humidity of the test environment is 90% RH or above, so that even if the relative humidity of the test environment changes by ±5% RH, it will not affect the experimental results.

[0051] Furthermore, in the corrosion process, in order to maintain the state of the solution supplied in the supply process, a solution-holding substance containing the solution is disposed on the end face of the metal material. Thus, the state of continuous wetting in the end face of the metal material can be reliably maintained. The solution-holding substance only needs to have voids in the substance, such as cotton gauze, mud, etc., and be able to maintain the state of holding the solution by capillary action. It is preferred that the solution-holding substance is a substance that allows oxygen to permeate without hindering corrosion. It should be noted that in order to keep the wetting state of the end face of the metal material constant, it is preferred that the solution-holding substance does not move during the test. Additionally, the supply process can be carried out only once at the beginning, or the supply process and the corrosion process can be repeated. When the supply process and the corrosion process are repeated, it is preferred that the corrosion process is carried out for 100 hours or more.

[0052] Example 1

[0053] First, verify whether the delayed fracture test method of the present invention simulates actual corrosion through the following Example 1. First, as the metal materials for forming the test pieces, steel grades A and B with the compositions shown in Table 1 are used.

[0054] [Table 1]

[0055]

[0056] Figure 1 is a schematic diagram showing an example of a test piece for delayed fracture evaluation. As Figures 1 to 3 shown, a steel plate with a thickness of 1.4 mm formed of steel grades A and B is sheared into a width of 32.5 mm × a length of 100 mm to serve as a test piece 1 for the test. Thus, the test piece 1 has a sheared end face 2. It should be noted that the side other than the evaluation end face is ground to a width of 30 mm. In addition, hole machining for bolts is performed at a position separated from the center of the plate.

[0057] The above-mentioned test steel plate is immersed in toluene and ultrasonically cleaned for 5 minutes, then 180° bending processing is performed. After springback, the test piece 1 is constrained using bolts BN and nuts NN to fabricate the test piece 1. This test piece 1 for delayed fracture evaluation has a bent portion with a bending radius R = 7 mm, and an arbitrary stress is supplied to the bending top by adjusting the tightening amplitude of the bolts BN and nuts NN. Under the condition of more tightening, the load stress becomes larger, which is a severe condition. The stress at the top generated by tightening is denoted as the tightening stress. In this embodiment, it is set to five levels of 800, 1000, 1200, 1400, and 1600 MPa. The tightening stress is determined by estimating the tightening amplitude based on the CAE analysis of the SS curve for each material.

[0058] For the test pieces 1 using steel grades A and B, both the actual environment test of conducting a cracking test in the actual environment of an actual snow-melting salt spreading area and the test based on the delayed fracture evaluation method of the metal material of the present invention are carried out.

[0059] <Actual environment test>

[0060] In the actual environment test, each test piece 1 is set under the lower part of a moving body that travels on a road in a state where snow-melting salt is scattered every day, and is recovered on the 60th day after the start of the test. The reason for setting it under the moving body is that the lower part of the moving body is the part affected by the snow-melting salt. The results are shown in Table 2 below. It should be noted that the minimum load stress at which cracking occurs during the test period is set as the boundary at which cracking occurs, and is used as the ultimate load stress in the actual environment.

[0061] [Table 2]

[0062]

[0063] In Table 2, the test piece conditions where cracks of 1 mm or more appeared on Test Piece 1 were set as having cracking (symbol: ×), and the conditions of cracks less than 1 mm or no cracks were set as no cracking (symbol: 〇). Figure 2 It is a schematic diagram for discriminating the location observed from the top of the U-bend test piece. As Figure 2 shown, discrimination is performed using the cracking (crack) CK on the end face 2 side of the evaluation. In addition, when the minimum stress of the load stress in Test Piece 1 where cracking CK occurs is defined as the ultimate load stress, the ultimate load stress of Steel A is 1400 MPa, and the ultimate load stress of Steel B is 1000 MPa.

[0064] <Delayed fracture property test>

[0065] Next, as Figure 3 shown, the above-described delayed fracture evaluation method for metallic materials is implemented using the same Test Piece 1 as in the above actual environment test, and the solution holding member 10 is disposed on the cross-section 2 during the corrosion process. At this time, the test period is set to a maximum of 60 days, and the minimum load stress at which cracking occurs during the test period is set as the boundary at which cracking occurs, as the ultimate load stress. And by comparing the ultimate load stress and the corrosion mode (corrosion type) obtained in the delayed fracture property test with the ultimate load stress and the corrosion mode (corrosion type) obtained in the above actual environment test, it is determined whether the delayed fracture property test is appropriate. It should be noted that the relative humidity in the atmospheric atmosphere within the range of ±5% including the set value is recognized as the test range.

[0066] The test conditions and results of the delayed fracture evaluation test are shown in Table 3.

[0067] [Table 3]

[0068]

[0069] It should be noted that in Table 3, the cases where the test conditions are based on an embodiment of the present invention are taken as examples, and the cases where the test conditions are not satisfied are taken as comparative examples. The parts outside the numerical range of this application are underlined. In addition, regarding the condition where none of the test pieces cracked, the ultimate load stress is recorded as 1600 MPa or more. The conditions inconsistent with the actual environmental test are set as × (comparative example), the conditions with consistent results are set as "B", and the more preferable results among them are set as "A". Specifically, as described above, from the viewpoints of the ultimate load stress and the corrosion mode (corrosion type), the evaluation of the matching with the actual environment is carried out. And when both the ultimate load stress and the corrosion mode (corrosion type) are consistent with the actual environment, it is set as "A". In addition, when the ultimate load stress is consistent but the corrosion mode is different, since it is established as a delayed fracture evaluation test, it is set as "B". On the other hand, when both the ultimate load stress and the corrosion mode (corrosion type) are inconsistent with the actual environment, it is set as "×" as an inappropriate delayed fracture evaluation test.

[0070] Examples No. 3, 5, 13, 23, 26 are examples in which steel type A is used, and the liquid film thickness is changed by fixing the conditions other than the chloride amount and changing the chloride amount. In the environment of No. 3, since the supply amount of the chloride is small, the liquid film is not fully formed, and the corrosion is less than that of the actual environmental test. Therefore, the cracking result (ultimate load stress) is consistent with the actual environment but slow. In addition, in No. 26, the chloride amount is large and the liquid film thickness is too thick. Therefore, the ultimate load stress is consistent with the actual environment, but a corrosion mode different from the actual environment is exhibited. Examples No. 30, 32, 40, 50, 53 are examples using steel type B and show the same tendency.

[0071] Examples and comparative examples of the cases where steel types A and B are used and the liquid film thickness is changed by changing the amount of chloride and the relative humidity of the test environment are No. 1, 2, 4, 13, 28 - 29, 31, 40. The formed liquid film thickness is determined by the relationship between the relative humidity of the test environment and the chloride amount. In No. 4, 31, the humidity is 75% RH and the chloride amount is 10000 mg / m 2 , so the state where the liquid film can be kept existing without interruption (the liquid film thickness is 10 μm or more) can be maintained. In No. 1, 28 as comparative examples, the relative humidity of the test environment is low and lower than the relative humidity for moisture absorption by the chloride contained in the solution. Therefore, the liquid film is basically not formed and the corrosion does not occur. Therefore, the environment is different from the actual environment. In addition, in the examples of No. 13, 40 where the relative humidity of the test environment is 90% or more, the water film thickness is within an appropriate range and the corrosion condition is also similar to the actual environment. Therefore, more preferable results are obtained.

[0072] Examples and comparative examples numbered 13, 22, 27, 40, 49, and 54 involve changing the type of solution. Examples numbered 13, 22, 40, and 49 contain chloride solutions, so the results are consistent with the actual environment. Comparative examples numbered 27 and 54 are solutions without chloride ions. Since they do not contain chloride ions, a uniform liquid film is not formed and the results are inconsistent with the actual environment.

[0073] Examples numbered 8 - 10, 13, 19 - 21, 35 - 37, 40, 46 - 48 involve changing the test temperature. Examples numbered 9, 10, 13, 19, 20, 36, 37, 40, 46, and 47 show a high degree of consistency with the actual environment. In Examples numbered 8 and 35, the solution froze due to the low temperature and corrosion no longer occurred midway. Although the cracking results (ultimate load stress) are consistent with the actual environment, the corrosion mode is inconsistent. In cases like Examples numbered 21 and 48, if the temperature is too high, the corrosion proceeds above the actual environment. Although the cracking results (ultimate load stress) are consistent with the actual environment, the corrosion mode is inconsistent.

[0074] Examples and comparative examples numbered 11 - 13, 18, 38 - 40, 45 involve changing the pH of the solution. Examples numbered 12 - 13, 18, 39 - 40, 45 are highly consistent with the actual environment. In comparative examples numbered 11 and 38, the results are more severe than the actual environment. This is because a low solution pH promotes iron dissolution, and as the iron dissolves, hydrogen ions in the solution are reduced, promoting hydrogen intrusion into the steel. Therefore, it is expected to be more severe than the actual environment and is inconsistent with the actual environment.

[0075] Examples and comparative examples numbered 5, 7, 13, 16 - 17, 23, 25, 32, 34, 40, 43 - 44, 50, 52 involve changing the substances in the solution. Examples numbered 5, 7, 13, 16, 23, 25, 32, 34, 40, 43, 50, 52 are cases where the substances in the solution are changed. In these examples, by configuring substances to maintain the solution, the liquid film thickness is maintained, and thus the results are consistent with the actual environment. Since the solution cannot be maintained if substances with water - retaining ability are not configured as in Examples numbered 17 and 44, the progress of corrosion is different and it is inconsistent with the actual environment.

[0076] Examples numbered 5 - 6, 13 - 15, 23 - 24, 32 - 33, 40 - 42, 50 - 51 involve changing the supply method of the solution. Examples numbered 5 - 6, 13 - 15, 23 - 24, 32 - 33, 40 - 42, 50 - 51 are examples that are consistent with the actual environment even when the supply method is changed under the condition of meeting the liquid film thickness.

[0077] Example 2

[0078] Next, regarding whether the difference in the end face leads to different evaluations of delayed fracture, verification was carried out according to the following Example 2. For test pieces processed by three processing methods of shearing, laser cutting, and grinding the end face after shearing using steel type A with a thickness of 1.4 mm, the same U-bending process as in Example 1 was performed, and each end face was evaluated using the present invention. The results are shown in Table 4.

[0079] [Table 4]

[0080]

[0081] As shown in Table 4, it can be seen that when evaluated using the present invention, even when using the same steel type A, the cracking results differ depending on the state of the end face. That is, by using the present invention, it is possible to evaluate which end face state is effective from the viewpoint of delayed fracture characteristics.

[0082] The embodiments of the present invention are not limited to the above embodiments, and various modifications can be made. For example, the metal material to be evaluated is usually a steel material such as a steel plate, but is not limited thereto, and may also be a metal material such as Ti or Al. The delayed fracture characteristic evaluation method of the present invention can accurately evaluate the delayed fracture characteristics of metal materials, and thus the selected metal materials (especially steel materials such as steel plates) have excellent delayed fracture characteristics.

[0083] Explanation of Reference Numerals

[0084] 1 Test piece

[0085] 2 End face

[0086] BN Bolt

[0087] NN Nut

[0088] 10 Solution holding substance

Claims

1. Method for evaluating the delayed fracture characteristics of a metallic material, which is a method for evaluating the delayed fracture characteristics of the end face of a metallic material, wherein, On the end face, a solution holding material containing a solution with a pH of 3.5 or higher and containing a chloride is disposed on the end face, and the liquid film thickness of the solution is maintained at 10 μm or more and 2500 μm or less, and the state maintained at the deliquescence humidity of the chloride is continued to corrode the end face. The solution holding material is a material capable of maintaining the state of holding the solution by capillary action.

2. The method for evaluating the delayed fracture characteristics of a metallic material according to claim 1, wherein, Corrosion is carried out at a test temperature of -50 to 60 °C.

3. The method for evaluating the delayed fracture characteristics of a metallic material according to claim 1 or 2, wherein, After supplying a solution with a pH of 3.5 or higher and containing a chloride to the end face, the solution holding material is disposed on the end face.

4. The method for evaluating the delayed fracture characteristics of a metallic material according to claim 3, wherein, The supply of the solution is carried out by any one of spraying, spraying, dropping of liquid droplets, or immersion for less than 15 minutes.

5. The method for evaluating the delayed fracture characteristics of a metallic material according to any one of claims 1 to 4, wherein, The metal material is a steel plate with a tensile strength of 1180 MPa or higher.

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